High-frequency dielectric heating apparatus



June 14, 1949. E, T. Hsu

Y HIGH-FREQUENCY DIELECTRIC HEATING APPARATUS Filed May 29. 1945 Inventor: Eugene'l'f l-ls by His Attorney.

Patented June 14, 1949 HIGH-FREQUEN CY DIELECTRIC HEATING APPARATUS Eugene T. Hsu, New York, N. Y., assignor to General Electric Company, a corporation of New York Application May 29, 1945, Serial No. 596,575

2 Claims. 1

My invention relates to high frequency heating apparatus, more particularly to high frequency dielectric heating apparatus for the continuous heating of elongated material, and has for its object simple and reliable high frequency heating apparatus provided with means for preventing electric arcing and corona discharge from the electrodes.

My invention is useful in the heating of materials which evolve an appreciable amount of vapor more conductive electrically than air when the materials are heated. Such vapor evolving conditions are found in the heating for drying or curing purposes of various elongated materials which are passed continuously in the form of strips, threads, or cords between spaced elec trodes supplied with high frequency current at a suitable voltage. Such materials are textiles, rayon, rubber, plastics, paper and the like.

I have found that the water or other vapor evolved in the drying of a material by passing it continuously between two electrodes induces arcing between the electrodes and corona discharges from the electrodes, both of which effects are very undesirable because of the resulting power loss which decreases the efiiciency and effectiveness of the heating apparatus and moreover the arcing and corona discharge may result in damage to the material being heated.

In carrying out my invention in one form I place a layer of vitreous electrically insulating material having a high dielectric constant, such as quartz or Pyrex glass, between each electrode and-the material being heated in such manner that the electrodes are prevented by the dielectric material from coming into direct contact with the material being heated or exposed to the vapor evolvedfrom the material. The dielectric material may be arranged to surround each electrode, or it may take the form of tubes through which the threads or cords are passed with the electrodes on the outsides of the tubes, or plates on opposite sides of the material being heated and between the electrodes and the material being heated.

For more complete disclosure of my invention reference should be made to the accompanying drawing, Fig. 1 of which is a fragmentary view in perspective of high frequency heating apparatus embodying my invention; Fig. 2 is an enlarged view, partly in section, of one of the electrodes shown in Fig. 1; Fig. 3 is an end view of the electrode shown in Fig. 2, while Figs 4 and 5 are simplified views in perspective showing modified forms of my invention.

I have shown in Fig. 1, one form of my invention as applied to the heating for drying pur poses of warp threads on a slasher. In passing through the slasher the plurality of parallel warp threads, indicated as a band or strip l are passed through a suitable container (not shown) containing a sizing material, such as water with a small amount of gelatin, after which the warp is passed through rolls (not shown) for the removal of the excess liquid and then heated to drive off the remaining liquid.

For purposes of simplification, I have shown the warp threads I as being passed between two rolls 2 and 3. It will be understood that suitable driving means (not shown), is provided for one of the rolls for winding the warp on that roll and drawing it oif the other roll. The warp threads may be made of rayon, cotton or other suitable material.

In accordance with my invention, I provide upper and lower groups of electrodes 4 and 5 adjacent respectively the upper and lower sides of the warp although preferably the spacing is sufficient to prevent the touching of the electrodes by the warp threads and the deposit thereby of the sizing material on the electrodes. The electrodes in each group are arranged in parallel relation and extending crosswise with respect to the warp and the electrodes of one group are in staggered relation with respect to the electrodes of the other group. As shown, the upper group of seven electrodes are supported on an electrically conducting rod 6 mounted on electrically insulating supports I and 8 while the lower group of six electrodes are supported on an electrically conducted rod 9 mounted on electrically insulated supports 10 and H. The electrodes of each group are, in addition, electrically connected to their supporting rods and by means of connections 12 and [3 the two groups are connected respectively to the terminals of a high frequency supply source (not shown) such as an electronic oscillator supplying current at a suitable voltage andfrequency. In typical heating apparatus as shown in Fig. 1 I use volt-- ages of approximately 5000 volts and frequencies of five to twenty-five megacycles.

For the purpose of protecting the electrodes from the water vapors driven off from the warp during the drying operation I enclose each electrode in a tube I4 made of quartz or Pyrex glass, as shown in detail in Figs. 2 and 3. The electrode itself is an inner tube 15 made of a suitable conductive material such as copper which tube is mounted inside of the outer insulation tube [4. Preferably as an important further preventive of both corona discharge and arcing, the electrode tube I5 is made somewhat smaller than the outer tube so that a peripheral space is provided between them. The inner electrode tube is held concentric with the insulating tube by a surrounding layer ll at each end of spacing material such as glass wool.

As shown clearly in Figs. 2 and 3, the electrode [5 is supported on a rod or stiff Wire I8 made of electrically conducting material whose ends l9 and 20 are secured, as by brazing, to opposite ends of the tube l5 and Whose center provided with a loop 2| is secured by the clamping screw 22 to an electrically conducting clamp 23. The clamp 23 is in turn secured by screw 24 to the supporting rod 6 or 9 as the case may be.

Preferably, as shown in the drawing, the electrode tubes are somewhat longer than the width of the warp I to prevent damaging arcing and corona discharge from the ends of the tubes which as shown are not covered by the layer of insulation l4. Moreover, the ventilation carrying away the vapors is more efficient at the exposed ends of the electrodes so that arcing and corona discharge are not likely to occur at the ends. Preferably forced ventilation as by a fan (not shown) is provided. Moreover, the ends of each tube l5 may be rounded to prevent arcing and corona discharge. For example each end may be closed by a hemispherical portion integral with the tube.

I have found that with the electrodes I5 enclosed by the quartz or Pyrex tubes 14 the same heating effect in the warp is obtained by a very substantially smaller power consumption as compared with the power consumption with bare electrodes. In addition to the power saving resulting from the elimination of radiation losses by arcing and corona, the quartz or Pyrex tube [4 because of its high dielectric constant of from 4 to 5 as compared with air provides a lower reluctance path than air for the electric field flux produced between the electrodes. Because of its low dielectric power loss factor, no substantial amount of heat is generated in the tube [4 and no substantial heating of the tube I4 occurs other than by the heat received by it from the warp l.

The efficiency of the heater is furthermore increased by the staggered relation of the electrodes of the upper and lower groups, 1. e., one group being in staggered relation with respect to the other. In this arrangement the electric field appears between each bottom electrode and the two top electrodes between which the bottom electrode is positioned and this electric field extends substantially along the length of the warp so as substantially to surround the warp throughout its entire length.

I contemplate that to prevent interference with radio communicaiton by the high frequency waves radiated from the heating apparatus, the apparatus will be enclosed in a field or enclosure (not shown) made of electrically conducting material and provided with a slot at each end for the passage of the warp I. For the purpose of preventing radiation through these slots I preferably provide a ground connection 24a for the group of electrodes having the odd number, i. e., the group 4.

In Fig. 4 I have shown a modified form of my invention for heating separate threads or cords. As shown, I provided three tubes 25, 26 and 21, made of quartz or Pyrex glass, through which the threads 28, 29 and 30 are passed continuously as by passing each from one spool to another. For heating the threads I provided a plurality of electrodes 3I35, inclusive, each of which is provided with three holes or apertures through which extends one of the tubes 25, 26 and 21. The electrodes are suitably spaced apart in conformity with the voltage and frequency of the supply source. Preferably the electrodes are connected in staggered relation, as in Fig. 1, to the supply source, the electrodes 3|, 33 and 35 being connected to one terminal, preferably grounded, of the supply source and the electrodes 32 and 34 being connected to the other terminal of the supply source.

Fig. 5 shows a still further modified form of my invention in which the strip of material 36 is passed between two plates 3'! and 38 made of electrically insulating material having a high dielectric constant. Electrodes 39 and 40 are arranged on opposite sides of the plates 31 and 38 whereby the electrodes are protected by the plates from vapors given off by the heated strip 36.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A high frequency heater for drying material impregnated with a vaporizable liquid, two elongated electrodes made of electrically conducting material, electric connections for supplying high frequency current to said electrodes, means for passing the material to be dried between said electrodes whereby said material is heated and the liquid driven off in the form of vapor, an outer tube of electrically insulating material surrounding each of said electrodes, and electrically insulating spacing means for securing each of said outer tubes in spaced relation with its said electrode thereby to protect said electrodes from the vapor driven olf from the material and prevent corona discharge and arcing from said electrodes.

2. A high frequency heater for drying elongated material impregnated with a vaporizable liquid, two cylindrical electrodes made of electrically conducting material, electric connections for supplying high frequency current to said electrodes, means for passing the material to be dried between said electrodes whereby said material is heated and the liquid driven off in the form of vapor, an outer tube of vitreous electrically insulating material having a high dielectric constant surrounding each of said electrodes, each of said outer tubes having an internal diameter substantially larger than the diameter of its said electrode, and electrically insulating spacing means for securing each of said tubes in spaced concentric relation with its said electrode thereby to protect the electrodes from the vapor driven 5 off from the material and prevent corona dis- Number charge and arcing from said electrodes. 2,163,898 EUGENE T. HSU. 2,204,603 2,231,457 REFERENCES CITED 5 2 23 2 9 The following references are of record in the 2,291,307 file of this patent: I UNITED STATES PATENTS 2:3 1:27 Number Name Date 9 1,927,381 Allen et a1. Sept. 19, 1933 1,972,050 Davis Aug.28,1934 Number 2,054,937 Kremer .Sept. 22, 1936 691,038

Name Date Van Der Lande June 27, 1939 Kline et a1. June 18, 1940 .Stephen Feb. 11, 1941 Crandell June 30, 1942 Hart, Jr Aug. 4, 1942 Brown Dec. 1, 1942 Baker et a1. Aug. 7, 1945 Gregory et a1. Aug. 7, 1945 FOREIGN PATENTS Country Date Germany May 15, 1940 

